They are less viscous and burn more easily (very flammable), they also burn with clean flames, producing little smoke and therefore making them very useful fuels.

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How is simple distillation done?

In steps by heating the mixture to different temperatures.

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How is fractional distillation done?

By vaporising the mixture and condensing the fractions at different temperatures.

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Where is crude oil separated into fractions?

At refineries using fractional distillation.

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How can fractional distillation separate crude oil into fractions?

The boiling point of a hydrocarbon depends on the size of its molecule.

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What does the boiling point of a hydrocarbon depend on?

The molecule size, the larger the molecule, the higher the boiling point of the hydrocarbon.

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What are stages of fractional distillation?

The crude oil is vaporised and fed into a fractionating column, the vapours move up - getting cooler as they rise, then they condense to liquids when they reach their boiling point, different liquids collect on trays at different levels.

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What is a fractionating column?

This is a tall tower that is hot at the bottom and gets cooler going up the column.

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How are the vapours able to travel up the column?

Inside the column there are many trays with holes to allow gases through.

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How are the different liquids separated inside the fractionating column?

Different liquids collect on the trays at different levels according to their boiling point and there are outlets to collect the fractions.

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What kind of hydrocarbons would be collected from the top of the column?

Hydrocarbons with the smallest molecules have the lowest boiling points and so are collected at the top, the fractions collected at the bottom of the column contain hydrocarbons with the highest boiling points.

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Why are different hydrocarbons collected at different levels of a fractional distillation column?

Different hydrocarbons have different boiling points (so they condense at different levels).

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What properties would you expect for a fraction that is collected one-third of the way up a fractionating column?